Hydrolysable and polymerizable oxetane silanes

ABSTRACT

Polymerizable and hydrolysable oxetane silanes and in particular silicic acid condensates which can be prepared therefrom are described, which exhibit only a low volume shrinkage on polymerization and produce polymers with high mechanical strength and therefore can be used above all as dental material or constituent thereof.

This application claims the priority benefit of provisional applicationNo. 60/052,563 filed Jul. 15, 1997.

The invention relates to hydrolysable and polymerizable oxetane silanes,a process for the preparation thereof, silicic acid condensates,polymers and compositions prepared therefrom and the use of all thesematerials inter alia for the preparation of macromolecular compositionsby polymerization and for the preparation of composite materials,adhesives, coatings and in particular dental materials.

Hydrolysable silanes, which contain polymerizable organic radicals, areused in the preparation of coatings, particulate fillers, adhesivecompositions and monolithic moulded articles and in the surfacemodification of reinforcing substances. The silanes are hydrolyticallycondensed and polymerized thermally, photochemically or by redoxinitiation, i.e. cured, alone, mixed with other silanes or in thepresence of other metal alkoxides.

Of particular interest in connection with the preparation oforganic-inorganic composite materials are above all organically modifiedsilanes with polymerizable organic groups, such as vinyl, (meth)acrylic,allyl or styryl groups, since they permit the simultaneous orconsecutive formation both of an inorganic and of an organic network andtherefore of composite materials with customized properties (cf H.Schmidt, Mat. Res. Soc. Symp. Proc. Vol. 32 (1984), 327-335; H. Schmidt,H. Wolter, J. Non-Cryst. Solids 121 (1990) 428-435). The polymerizablesilanes are as a rule in the first step hydrolytically condensed insolution. After the addition of thermal initiator or photoinitiator andremoval of the solvent, nanoparticulate resins then form which areshaped and then polymerized and thus cured.

A major disadvantage of these materials, however, is that thedevelopment of the organic network which takes place on polymerizationis mostly accompanied by a considerable volume contraction which mayresult in deformation of the moulded articles, reduction in substrateadhesion, layer separation, development of voids or development ofmaterial stresses. A reduced volume contraction takes place with silaneswhich bear ring-opening groups. In this connection, EP-B-0 358 011describes scratch-resistant materials inter alia based on3-glycidyloxypropyl silanes, EP-B-0 486 469 describes organic-inorganichybrid polymers of 3-glycidyloxypropyl silanes and DE-C-41 33 494describes dental resin compositions in which e.g. silanes withring-opening spiroortho ester groups are used. It proves to bedisadvantageous, however, that epoxide silanes are toxicologicallyunacceptable and cationically polymerize sufficiently quickly atelevated temperatures only. Furthermore, spiroortho ester silanesexhibit only a low stability and their cationic ring-openingpolymerization is generally accompanied by the formation of lactone.

Furthermore, the following silicon-containing oxetane derivatives arealso known:

1. Silicon-containing oxetanes which are obtainable e.g. byhydrosilylation of 3-allyloxymethyl-3-ethyl-oxetane with1,1,3,3-tetramethyldisiloxane (cf J. V. Crivello et al., J. Macromol.Sci.-Pure Appl. Chem. A30 (1993), 173-187): ##STR1## 2.3-(trimethylsiloxy)-oxetanes which can be synthesized by Paterno-Buchireaction (cf T. Bach, Tetrahedron Lett. 32 (1991), 7037-8): ##STR2## 3.3-Alkyl-3-(triorganosiloxymethyl)-oxetanes or3-alkyl-3-(triorganosilylmethyl)-oxetanes (cf DE-A-195 06 222): ##STR3##4. 3,3-bis(triorganosiloxymethyl)-oxetanes which can be obtained byreacting 3,3-bis(hydroxymethyl)-oxetanes with appropriatetriorganoaminosiloxanes R₃ SiNH₂ (cf Chem. Abstr. 76 (192) 14701k):##STR4##

It is the object of the invention to provide hydrolysable andpolymerizable oxetane silanes from which, alone or together with otherhydrolytically condensable and polymerizable components, stablecompositions can be prepared which polymerize with only low shrinkageand at high speed at room temperature, and which are suitable ascomposite or coating material, adhesive or adhesion promoter or for thepreparation of fillers or materials for medical or dental purposes.These silanes are to be able to be covalently incorporated intoorganic-inorganic composite materials and be synthetically obtainable sothat the distance between silicon and the polymerizable groups can bevaried.

This object is achieved according to the invention by the hydrolysableand polymerizable oxetane silanes according to Claims 1 to 3. Theinvention further relates to the silicic acid condensates according toClaim 4, the polymerizates according to Claim 5, the compositionsaccording to Claims 6 and 7 and the use according to Claim 8.

The hydrolysable and polymerizable oxetane silanes according to theinvention and the stereoisomers thereof correspond to the generalformula (I): ##STR5## in which the variables R⁰, R¹, R², R³, R⁴, R⁵, R⁶,X, Y, a, b, c, and x, unless otherwise stated, independently of oneanother have the following meanings:

R⁰ =hydrogen or substituted or unsubstituted C₁ to C₁₀ alkyl;

R¹ =missing or represents substituted or unsubstituted C₁ to C₁₈alkylene, C₆ to C₁₈ arylene, C₇ to C₁₈ alkylenearylene orarylenealkylene, these radicals being able to be interrupted by at leastone group selected from ether, thioether, ester, carbonyl, amide andurethane groups;

R² =missing or represents substituted or unsubstituted C₁ to C₁₈alkylene, C₆ to C₁₈ arylene, C₇ to C₁₈ alkylenearylene or C₇ to C₁₈arylenealkylene, these radicals being able to be interrupted by at leastone group selected from ether, thioether, ester, thioester, carbonyl,amide and urethane groups or being able to bear these in the terminalposition;

R³ =missing or represents substituted or unsubstituted C₁ to C₁₈ alkyl,C₂ to C₁₈ alkenyl, C₆ to C₁₈ aryl, C₇ to C₁₈ alkylaryl or C₇ to C₁₈arylalkyl, these radicals being able to be interrupted by at least onegroup selected from ether, thioether, ester, carbonyl, amide andurethane groups;

R⁴ =missing or represents substituted or unsubstituted --CHR⁶ --CHR⁶ --,--CHR⁶ --CHR⁶ --S--R⁵, --S--R⁵ --, Y--CO--NH--R⁵ -- or --CO--O--R⁵ --;

R⁵ =substituted or unsubstituted C₁ to C₁₈ alkylene, C₆ to C₁₈ arylene,C₆ to C₁₈ alkylenearylene or C₆ to C₁₈ arylenealkylene, these radicalsbeing able to be interrupted by at least one group selected from ether,thioether, ester, carbonyl, amide and urethane groups;

R⁶ =hydrogen or substituted or unsubstituted C₁ to C₁₈ alkyl or C₆ toC₁₀ aryl;

X=a hydrolysable group, namely halogen, hydroxy, alkoxy or acyloxy;

Y=O or S;

a=1, 2 or 3;

b=1, 2 or 3;

c=1 to 6; and

x=1, 2 or 3; and with the proviso that

(i) a+x=2, 3 or 4 and

(ii) a and/or b=1.

However, the above formula covers only those compounds which arecompatible with the valency theory.

The silanes according to the invention are usually present asstereoisomer mixtures and in particular as racemates.

The ether, thioether, ester, thioester, carbonyl, amide and urethanegroups which are possibly present in the radicals are defined by thefollowing formulae: --O--, --S--, --CO--O--, --O--CO--, --CO--S--,--S--CO--, --CS--O--, --O--CS--, --CO--, --CO--NH--, --NH--CO--,--O--CO--NH--and --NH--CO--O--.

The non-aromatic radicals or non-aromatic parts of the radicals possiblein formula (I) can be straight-chained, branched or cyclic.

Alkyl radicals have preferably 1 to 8 and particularly preferably 1 to 4carbon atoms. Particular examples of possible alkyl radicals are methyl,ethyl, n- and iso-propyl, sec- and tert-butyl, n-pentyl, cyclohexyl,2-ethylhexyl and octadecyl.

Alkenyl radicals have preferably 2 to 10 and particularly preferably 2to 6 carbon atoms. Particular examples of possible alkenyl radicals arevinyl, allyl and iso-butenyl.

Preferred examples of possible aryl radicals are phenyl, biphenyl andnaphthyl.

Alkoxy radicals preferably have 1 to 6 carbon atoms. Particular examplesof possible alkoxy radicals are methoxy, ethoxy, n-propoxy, iso-propoxyand tert-butoxy.

Acyloxy radicals preferably have 2 to 5 carbon atoms. Particularexamples are acetyloxy and propionyloxy.

Preferred alkylene radicals are derived from the above preferred alkylradicals and preferred arylene radicals are derived from the abovepreferred aryl radicals.

Preferred radicals consisting of a combination of non-aromatic andaromatic parts, such as alkylaryl, arylalkyl, alkylenearylene andarylenealkylene radicals, are derived from the above preferred alkyl andaryl radicals. Particular examples thereof are benzyl, 2-phenylethyl andtolyl.

The mentioned substituted R radicals bear one or more simplesubstituents. Examples of these substituents are methyl, ethyl, phenyl,benzyl, hydroxymethyl, hydroxyethyl, methoxy, ethoxy, chloro, bromo,hydroxy, mercapto, isocyanato, vinyloxy, acryloxy, methacryloxy, allyl,styryl, epoxy, carboxyl, SO₃ H, PO₃ H₂ or PO₄ H₂.

For a, b, c or x≧2, the radicals X and the individual R radicals can ineach case have the same or a different meaning.

Moreover, preferred definitions exist for the above-stated variables offormula (I) which, unless otherwise stated, can be chosen independentlyof one another and are as follows:

R⁰ =hydrogen or C₁ to C₅ alkyl;

R¹ =C₁ to C₈ alkylene, these radicals being able to be interrupted by atleast one group selected from ether, thioether, ester and urethanegroups;

R² =missing or represents C₁ to C₈ alkylene, these radicals being ableto be interrupted by at least one group selected from ether, thioether,ester, thioester, carbonyl, amide and urethane groups or being able tobear these in the terminal position;

R³ =missing or represents methyl, ethyl or phenyl;

R⁴ =missing or represents --CHR⁶ --CHR⁶ --, --S--R⁵ --, --Y--CO--NH--R⁵-- or --CO--O--R⁵ --;

R⁵ =C₁ to C₈ alkylene, these radicals being able to be interrupted by atleast one group selected from ether, thioether, ester, carbonyl, amideand urethane groups;

R⁶ =hydrogen or C₁ to C₅ alkyl;

X=methoxy, ethoxy or chloro;

Y=O or S;

a=1;

b=1;

c=1 to 6;

x=2 or 3; and/or

a+x=3.

The individual R radicals can in turn bear simple substituents.

Preferred compounds are accordingly those in which at least one of thevariables of formula (I) has the above-described preferred definition.

Furthermore, those oxetane silanes of formula (I) are preferred in whichthe indices a, b and/or c have the value 1, and examples thereof are thesilanes according to the general formulae (II), (III), (IV) and (V)below. ##STR6## Particular examples of preferred oxetane silanesaccording to the invention of formula (I) are given below: ##STR7##

The preparation of the oxetane silanes (I) according to the invention ispossible in particular via a large number of conventional addition orcondensation reactions which are carried out according to the methodscustomary for these reactions. Processes which can be used for preparingthe silanes according to the invention are described e.g. in W. Noll,Chemie und Technologie der Silicone, 2nd edition, Verlag Chemie,Weinheim 1968, in particular p. 22 et seq., and in the review by R. C.Mehrotra in J. Non-Crystalline Solids 100, (1988) 1-15 and theliterature quoted in this article.

In a first variant, e.g. 3-ethyl-3-hydroxymethyl oxetane (1) can beadded to an isocyanate group-containing silane: ##STR8##

Furthermore, starting from 3-acryloyloxymethyl-3-ethyloxetane (2), e.g.the thiol-ene addition with mercaptosilanes is possible: ##STR9##

Moreover, the semi-ester of fumaric acid and3-ethyl-3-hydroxymethyl-oxetane can be added to an epoxide silane:##STR10##

The silane obtained can be reacted further with an isocyanategroup-containing silane, so that oxetane silanes with two silyl groupsare obtained: ##STR11##

Silanes with several oxetane radicals are accessible via additionreactions. Thus, e.g. by reacting (1) with tetracarboxylic aciddianhydrides, such as pyromellitic acid anhydride,1,2,3,4-butanetetracarboxylic acid dianhydride ortetrahydrofuran-2,3,4,5-tetracarboxylic acid dianhydride, an adduct canbe obtained which is further reacted with 1 or 2 mol of an epoxide orisocyanatosilane. ##STR12##

The silanes (I) according to the invention are polymerizable via theoxetane groups and hydrolysable via the radicals X. The polymerizationof the oxetane groups leads to the formation of an organic network,whereas the hydrolysable groups produce an inorganic polysiloxanenetwork through polycondensation.

The oxetane silanes according to the invention are substances of highreactivity which on hydrolysis form polymerizable silicic acidcondensates which can be polymerized in the presence of usual cationicinitiators or photoinitiators at room temperature or under irradiationwith light of the visible or UV range to form mechanically stablelayers, moulded articles or fillers.

The number of hydrolysable groups, polymerizable groups and furtherfunctional groups can be varied by suitable selection of the startingmaterials used in the preparation of the oxetane silanes. Depending onthe type and number of the hydrolysable groups, e.g. alkoxy groups, andon the number of oxetane groups, the condensation of the oxetane silanesand the polymerization of the obtained condensates therefore results inmaterials with properties which range from silicone rubber-like toglass-like. In comparison with radically polymerizable silanes, noinhibition layer forms on polymerization of the oxetane silanesaccording to the invention, which is very advantageous especially in thepreparation of coatings.

The development of a three-dimensional, organic network is possible whenat least two oxetane radicals are present, the mechanical properties,such as e.g. strength and flexibility, and the physico-chemicalproperties, e.g. adhesiveness, water absorption and refractive index, ofthe obtained silicic acid condensates being variable and optimallyadaptable to the requirements of the respective case of application viathe distance between the Si atom and the oxetane radical, i.e. via thelength of the spacer group, and by incorporation of further functionalgroups. Aliphatic groups result in products which are rather flexible,and aromatic groups result in products which are rather rigid.

Furthermore, the crosslinking density, which then likewise influencesthe properties and possible applications of the corresponding silicicacid condensates, can be set by means of the number of polymerizableoxetane groups. Moreover, if the oxetane silanes according to theinvention also contain ionically crosslinkable groups such as e.g.(meth)acrylate, styryl or allyl, then a further increase in crosslinkingdensity can be achieved simultaneously or consecutively, i.e. as a2-stage process, by their radical polymerization.

The oxetane silanes according to the invention and their silicic acidcondensates possess only a low volatility, with the result that they canbe processed in an easy and largely harmless manner. In view of theabove-stated variation possibilities of the condensable andpolymerizable radicals of the oxetane silanes according to theinvention, silicic acid condensates which can be prepared therefrom canbe provided as resins or fillers for very different areas ofapplication.

The silanes (I) are stable compounds which can be processed either aloneor together with other hydrolysable, condensable and/or polymerizablecomponents to form the silicic acid condensates according to theinvention.

In addition to the silanes of formula (I), other hydrolyticallycondensable compounds of silicon, aluminium, titanium, zirconium orphosphorus can be used in the preparation of the silicic acidcondensates according to the invention, which are then also referred toas silicic acid (hetero)condensates. These compounds can be used eitheras such or in already precondensed form. It is preferred that at least20 mol. %, particularly preferably at least 80 mol. %, based onmonomeric compounds, of hydrolysable silicon compounds are used for thepreparation of the silicic acid (hetero)condensates according to theinvention. It is also preferred that at least 10 mol. %, in particular40 to 100 mol. %, in each case based on monomeric compounds, of oxetanesilanes according to the invention are used for the preparation of thesilicic acid (hetero)condensates.

Preferably at least one silane of the general formula (VI) is used asother hydrolytically condensable compounds:

    R.sup.7.sub.k (Z'R.sup.8).sub.m SiX'.sub.4-(k+m)           (VI)

in which R⁷, Z', R⁸, X', k and m, unless otherwise stated, independentlyof one another have the following meanings:

R⁷ =C₁ to C₈ alkyl, C₂ to C₁₂ alkenyl or C₆ to C₁₄ aryl;

R⁸ =C₁ to C₈ alkylene, C₂ to C₁₂ alkenylene or C₆ to C₁₄ arylene;

X'=hydrogen, halogen or C₁ to C₈ alkoxy;

Z'=mercapto, glycidyl, acrylic, methacrylic, vinyl, allyl or vinyl ethergroup;

k=0, 1, 2 or 3;

m=0, 1, 2 or 3; and

k+m=1, 2 or 3.

Such silanes are described e.g. in DE-C-34 07 087, and particularexamples of hydrolytically condensable silanes of general formula (VI)are:

CH₃ --Si--Cl₃, CH₃ --Si--(OC₂ H₅)₃, C₂ H₅ --Si--Cl₃, C₂ H₅ --Si--(OC₂H₅)₃, CH₂ ═CH--Si--(OC₂ H₅)₃, CH₂ ═CH--Si--(OCH₃)₃, CH₂ ═CH--Si--(OC₂ H₄OCH₃)₃, (CH₃)₂ --Si--Cl₂, (CH₃)₂ --Si--(OC₂ H₅)₂, (C₂ H₅)₃ --Si--Cl, (C₂H₅)₂ --Si--(OC₂ H₅)₂, (CH₃)₃ --Si--Cl, (CH₃ O)₃ --Si--C₃ H₆ --NH₂, (CH₃O)₃ --Si--C₃ H₆ --SH, (CH₃ O)₃ --Si--C₃ H₆ --NH₂, ##STR13##

Furthermore, at least one zirconium, titanium or aluminium compound ofthe formulae

    MeX"R.sup.9.sub.z AlR.sup.10.sub.3

can be used as other preferred hydrolytically condensable compounds, inwhich Me, R⁹, R¹⁰, X", y and z independently of one another have thefollowing meanings:

Me=Zr or Ti;

R⁹ =hydrogen, substituted or unsubstituted C₁ to C₁₂ alkyl, C₇ to C₁₅alkylaryl or C₆ to C₁₄ aryl;

R¹⁰ =halogen, OH, C₁ to C₈ alkoxy;

X"=halogen, OH, C₁ to C₈ alkoxy;

y=1 to 4, in particular 2 to 4;

z=1 to 3, in particular 0 to 2.

Preferred examples of zirconium and titanium compounds which can be usedare ZrCl₄, Zr(OC₂ H₅)₄, Zr(OC₃ H₇)₄, Zr(OC₄ H₉)₄, ZrOCl₂, TiCl₄, Ti(OC₂H₅)₄, Ti(OC₃ H₇)₄ and Ti(OC₄ H₉)₄. Preferred examples of aluminiumcompounds which can be used are Al(OCH₃)₃, Al(OC₂ H₅)₃, Al(OC₃ H₇)₃,Al(OC₄ H₉)₃ and AlCl₃.

Complexed Zr, Ti and Al compounds can also be used, in which inter aliaacids or β-dicarbonyl compounds can act as complexing agents.

Other hydrolysable compounds which can be used for preparing the silicicacid (hetero)condensates are e.g. boron trihalides, tin tetrahalides,tin tetraalkoxides and vanadyl compounds.

The silicic acid condensates according to the invention of the silanes(I) are obtained by hydrolysis of the hydrolysable groups X present,e.g. alkoxy groups, and by subsequent condensation, which results in theformation of an inorganic network of Si--O--Si units. The hydrolysis andcondensation usually take place in basic or acidic medium, a linking ofC═C double bonds which are contained in the silanes used being generallyundesired.

The silicic acid condensates according to the invention can also bepresent in incompletely hydrolysed and condensed form. In such cases,they are referred to as so-called precondensates.

The customary procedure in the preparation of the silicic acid(hetero)condensates according to the invention is that the silanes (I),optionally dissolved in a solvent, are reacted at room temperature orwith slight cooling and in the presence of a hydrolysis and condensationcatalyst with the necessary quantity of water, and the resulting mixtureis stirred for one to several hours. Coming into consideration assolvents are above all aliphatic alcohols, such as e.g. ethanol ori-propanol, dialkyl ketones, such as acetone or methyl isobutyl ketone,ethers, such as diethyl ether or tetrahydrofuran (THF), esters, such asethyl or butyl acetate, and mixtures thereof.

If the hydrolytic condensation is carried out in the presence ofreactive Zr, Ti or Al compounds, the addition of water should take placein stages at ca 0 to 30° C. It is usually favourable not to add water assuch, but to introduce it in the form of water-containing solvents, suchas aqueous ethanol, or by release via a chemical reaction, such as viaan esterification.

The hydrolysis and condensation preferably takes place in the presenceof a condensation catalyst, preference being given to proton- orhydroxyl ion-releasing compounds such as organic or inorganic acids orbases. Particularly preferred are volatile acids or bases, in particularhydrochloric acid or ammonia. It has proved to be worthwhile forhydrolysis and condensation to adopt procedures of sol-gel technology,as are described e.g. in C. J. Brinker et al., "Sol-Gel Science",Academic Press, Boston, 1990. The "sol-gel process" is also disclosed inDE-A-27 58 414, DE-A-27 58 415, DE-A-30 11 761, DE-A-38 26 715 andDE-A-38 35 968.

The obtained silicic acid (hetero)condensates of the silanes (I) andoptionally of other hydrolytically condensable compounds can be usedeither as such or after partial or complete removal of the solvent used.In some cases, it can also prove to be advantageous to replace thesolvent used for the hydrolytic condensation with another solvent.

The polymerizable silicic acid (hetero)condensates according to theinvention and the silanes (I) and compositions containing thesecondensates or silanes can be cured by cationic polymerization orphotopolymerization, the polymerization usually taking place aftersuitable initiators and other polymerizable components have been added.If different polymerizable groups, e.g. oxetane and (meth)acrylicgroups, are present, several curing mechanisms, e.g. cationic andradical polymerization, can be used simultaneously or in successivestages.

Cationic initiators and/or photoinitiators are preferably used toinitiate the cationic polymerization.

Preferred examples of cationic initiators are strong Bronsted and Lewisacids, e.g. sulphuric acid, trifluoroacetic acid, aluminium trichlorideor boron trifluoride.

Suitable photoinitiators are onium salts, triarylsulphonium salts,diaryliodonium salts, cyclopentadienyl iron (I) salts and isoquinolinesalts. Particularly suitable are a mixture of4-(diphenylsulphino)-phenylphenylsulphide-bis-hexafluoroantimonate andbis[4-(diphenylsulphino)-phenyl]sulphide-hexafluoroantimonate (CyraureUVI 6974, Union Carbide), bis[4-(diphenylsulphino)-phenyl]sulphidehexafluorophosphate (Degacure Kl-85, Degussa), diphenyliodoniumhexafluoroantimonate or hexafluorophosphate, and the (η⁵-2,4-cyclopentadien-1-yl)[1,2,3,4,5,6-η]-(cumene)-iron(I)-hexafluorophosphinecomplex (Irgacure 261, Ciba Geigy). The sensitivity can be increased inthe visible range by sensitizers, such as thioxanthone derivatives,camphor quinone, phenanthrenequinone or perylene. Furthermore, it alsoproves to be advantageous to carry out the photopolymerization in thepresence of radical photoinitiators, such as e.g. benzoin alkyl ethers,benzil dialkyl ketals or acrylic phosphine oxides.

In the compositions according to the invention, suitable polymerizablemono- or multifunctional monomers which are also referred to as diluentmonomers can also be present in addition to the silanes (I) or thecorresponding silicic acid (hetero)condensates.

Particularly suitable diluent monomers are oxetane group-containingmonomers, such as 3-ethyl-3-hydroxymethyloxetane (1),3,7-bis(3-oxetanyl)-5-oxa-nonane (3),3,3'-(1,2-ethanediylbis(oxymethylene))-bis(3-ethyloxetane) (4),3,3'-(1,10-decanediylbis(oxymethylene))-bis(3-ethyloxetane) (5),3,3'-(1,3-(2-methylenyl)propanediyl-bis(oxymethylene))-bis(3-ethyloxetane)(6) or 3,3,'-(1,4-xylenediylbis(oxymethylene))-bis(3-ethyloxetane) (7):##STR14##

These compounds are known (cf H. Sasaki, J. V. Crivello, J. Macromol.Sci.-Pure Appl. Chem. A29 (1992) 915-930; J. V. Crivello, H. Sasaki, J.Macromol. Sci.-Pure Appl. Chem. A30 (1993) 189-206).

Moreover, radically polymerizable diluent monomers such asmonofunctional (meth)acrylates, e.g. methyl (meth)acrylate, ethyl(meth)acrylate, butyl (meth)acrylate, benzyl (meth)acrylate, furfuryl(meth)acrylate or phenyl (meth)acrylate, and polyfunctional(meth)acrylates, e.g. bisphenyl-A-di(meth)acrylate, bis-GMA (an additionproduct of methacrylic acid and bisphenol-A-diglycidyl ether), UDMA (anaddition product of 2-hydroxyethyl methacrylate and 2,2,4-hexamethylenediisocyanate), di-, tri- or tetraethylene glycol di(meth)acrylate,decanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate,pentaerythritol tetra(meth)acrylate, butanediol di(meth)acrylate,1,10-decanediol di(meth)acrylate or 1,12-dodecanediol di(meth)acrylate,can also be used.

The silanes according to the invention, the silicic acid condensates orsilicic acid heterocondensates thereof and compositions containing themcan be used as such or in at least partially polymerized form asvarnishes for coating plastics, glass or other substrates, as fillers orbulk material for composites and for producing medical materials such ascontact lenses. They are, however, particularly preferably used asdental material or a constituent thereof.

The compositions according to the invention can also optionally containother additives, such as e.g. coloring agents (pigments or dyes),stabilizers, flavorants, microbiocidal active ingredients, flameproofingagents, plasticizers or UV absorbers.

Other preferred additives are fillers. Examples of preferred fillers arequartz, glass ceramic or glass powders, in particular barium orstrontium silicate glass powder, lithium-aluminium silicate glasspowder, silicon, zirconium or aluminium oxides, or mixtures thereof,finely divided silicas, in particular pyrogenic or precipitated silicas,and X-ray-opaque fillers, such as e.g. ytterbium trifluoride.

A particularly preferred composition according to the inventioncontains:

(a) 5 to 90, in particular 10 to 70 wt. %, relative to the composition,of silicic acid (hetero)condensate of a silane (I),

(b) 0 to 80, in particular 0 to 50 wt. %, relative to the composition,of diluent monomer,

(c) 0.1 to 5, in particular 0.2 to 2.0 wt. %, relative to thecomposition, of polymerization initiator, and/or

(d) 0 to 90, in particular 0 to 80 wt. %, relative to the composition,of fillers.

The compositions according to the invention are particularly preferablyused as dental cement, dental filling material or dental bonding forfilling materials. The compositions are used in particular by applyingthem to the area of a false or natural tooth to be treated and curing bypolymerization.

It proves to be a particular advantage of the compositions according tothe invention that on the one hand they exhibit only a lowpolymerization shrinkage and on the other hand they result in compositematerials with high mechanical strength. Such a combination ofproperties is of particular significance especially in the case ofdental materials.

The invention is explained in more detail below with reference toexamples.

EXAMPLE 1 Synthesis of 7-trimethoxysilyl-4-thia-heptanoicacid-(3-ethyl-oxetan-3-yl)methyl ester (8)

1st stage: 3-ethyloxetan-3-yl-methyl acrylate ##STR15## 62.3 g (688mmol) of acrylic acid chloride in 300 ml of diethyl ether were addeddropwise to an ice-cooled solution of 80 g (688 mmol) of3-ethyl-3-hydroxyethyl-oxetane and 77.3 g of collidine (688 mmol) in 400ml of diethyl ether. After 6 hrs' stirring at room temperature, theformed hydrochloride was filtered off, and the filtrate was washed withaqueous hydrochloric acid and with NaHCO₃ solution. After drying withanhydrous Na₂ SO₄ and additional stabilizing with hydroquinonemonomethyl ether, the solvent was drawn off at 120 mbar in a rotaryevaporator. After fractional distillation (b.p.₀.2 :56° C.), 57 g (50%yield) of a colourless, clear liquid were obtained.

¹ H-NMR: 5.6-6.6 (m, 3H, CH═CH₂), 3.4-4.7 (m, 6H, CH₂ O), 1.6-1.9 (q,2H, CH₂), 0.7-1.0 (t, 3H, CH₃) ppm.

IR (Film): 2965, 2874, 1728, 1408, 1268, 1194 cm⁻¹.

2nd stage: 7-trimethoxysilyl-4-thia-heptanoicacid-(3-ethyl-oxetan-3-yl)methyl ester) ##STR16## 17.0 g (0.1 mol) of3-ethyl-oxetan-3-yl-methyl acrylate were added in a dry andargon-flushed apparatus to 19.6 g (0.1 mol) of3-mercaptopropyl-trimethoxy-silane, and the whole was stirred for 48 hrsat room temperature. After all volatile constituents had been removed bydrying at 60° C. at 0.1 mbar, 30 g (81% yield) of a colourless liquidwere obtained.

¹ H-NMR: 4.2-4.5 (m, 6H, CH₂ O), 3.6 (s, 9H, CH₃ O), 2.3-2.9 (m, 6H, CH₂S, CH₂ C═O), 1.6-1.0 (m, 4H, CH₂), 0.8-1.0 (t, 3H, CH₃), 0.6-0.7 (t, 2H,CH₂ Si) ppm.

IR (Film): 2940, 2870, 1737, 1459, 1244, 1089 cm⁻¹.

EXAMPLE 2 Synthesis ofN-(3-triethoxysilylpropyl)-(3-ethyloxetan-3-yl)-methyl carbamate (9)##STR17## 11.6 g (0.1 mol) of 3-(3-ethyloxetanyl)methanol were addedunder argon in a dry apparatus to 24.7 g (0.1 mol) of 3-isocyanatopropyltriethoxysilane and 25 ml of anhydrous ether. After 20 mg of dibutyltindilaurate had been added, the mixture was stirred for 6 hrs at roomtemperature. After the solvent had been drawn off, 30 g (ca 85%) of acolourless liquid remained.

Elemental Analysis

C₁₆ H₃₃ NO₆ Si [363.5] Calc.: C 52.86 H 9.15 N 3.85 Found: C 51.52 H9.48 N 3.76.

¹ H-NMR: 5.6 (br, H, NH), 4.4-4.5 (q, 4H, CH₂ O), 4.2 (s, 2H, CH₂ O),3.6-3.8 (q, 6HCH₂ O), 3.0-3.3 (q, 2H, CH₂ N), 1.4-1.8 (m, 4H, CH₂),1.1-1.3 (t, 9H, CH₃), 0.9 (t, 3H, CH₃), 0.4-0.7 (t, 2H, CH₂ Si) ppm.

IR (Film): 3336, 2972, 2930, 2880, 1724, 1533, 1245, 1080 cm⁻¹.

EXAMPLE 3 Synthesis of 2-(3-triethoxysilylpropylthio) succinicacid-bis-[(3-ethyloxetan-3-yl)-methyl] ester (10) ##STR18## 8.4 g (26.9mmol) of fumaric acid-bis-[(3-ethyloxetan-3-yl)methyl] ester, 5.3 g(26.9 mmol) of 3-mercaptopropyltriethoxysilane and 0.33 g (1.3 mmol) ofdibenzoyl peroxide were stirred for 5 hours in 20 ml of toluene at 100°C. When the solvent was distilled off, a white deposit precipitatedwhich produced 10.2 g (70% yield) of (10) after filtering off anddrying. EXAMPLE 4 Preparation of a Silicic Acid Condensate Based onSilane (8)

20 mmol of silane (8) and 20 mmol of dimethyldimethoxysilane weredissolved in 50 ml of anhydrous ethanol. After a mixture of 50 mmol ofwater and 5 ml of ethanol and some drops of 0.1 molar ethanolic aceticacid solution had been added, the whole was heated under ref lux for 5hours and was stirred overnight. After removal of volatile components invacuo, the formed resin (7 g) could be used for a cationicpolymerization.

EXAMPLE 5 Preparation of a Dental Bonding

70 mg, i.e. 1 wt. %, of Cyraure UVI 6974 (Union Carbide) were added to amixture of 4 g of resin from Example 4 and 3 g of3,7-bis(3-oxetanyl)-5-oxanonane (3). The mixture was then cast as a filmand irradiated for 60 s in a dental irradiation device, namely Heliomat(Vivadent). A solid, well adhering film formed.

The densities of the starting resin and the polymerisate were determinedin each case according to the buoyancy method, the change in volume,i.e. the shrinkage, during the cationic ring-opening polymerisationbeing given by the density difference. The obtained ΔV value of only-4.1% was clearly lower than that for conventional bondings based onmethacrylate. For example, the volume shrinkage during the curing of thecommercially available bonding Heliobond (Vivadent) is 7.5%.

What is claimed is:
 1. A hydrolysable and polymerizable oxetane silaneof the general formula (I) and stereoisomers thereof ##STR19## in whichthe variables R⁰, R¹, R², R³, R⁴, R⁵, R⁶, X, Y, a, b, c, and x, unlessotherwise stated, independently of one another have the followingmeanings:R⁰ =hydrogen or substituted or unsubstituted C₁ to C₁₀ alkyl;R¹ =missing or represents substituted or unsubstituted C₁ to C₁₈alkylene, C₆ to C₁₈ arylene, C₇ to C₁₈ alkylenearylene orarylenealkylene, these radicals being able to be interrupted by at leastone group selected from ether, thioether, ester, carbonyl, amide andurethane groups; R² =missing or represents substituted or unsubstitutedC₁ to C₁₈ alkylene, C₆ to C₁₈ arylene, C₇ to C₁₈ alkylenearylene or C₇to C₁₈ arylenealkylene, these radicals being able to be interrupted byat least one group selected from ether, thioether, ester, thioester,carbonyl, amide and urethane groups or being able to bear these in theterminal position; R³ =missing or represents substituted orunsubstituted C₁ to C₁₈ alkyl, C₂ to C₁₈ alkenyl, C₆ to C₁₈ aryl, C₇ toC₁₈ alkylaryl or C₇ to C₁₈ arylalkyl, these radicals being able to beinterrupted by at least one group selected from ether, thioether, ester,carbonyl, amide and urethane groups; R⁴ =missing or representssubstituted or unsubstituted --CHR⁶ --CHR⁶ --, --CHR⁶ --CHR⁶ --S--R⁵,--S--R⁵ --, --Y--CO--NH--R⁵ -- or --CO--O--R⁵ --; R⁵ =substituted orunsubstituted C₁ to C₁₈ alkylene, C₆ to C₁₈ arylene, C₆ to C₁₈alkylenearylene or C₆ to C₁₈ arylenealkylene, these radicals being ableto be interrupted by at least one group selected from ether, thioether,ester, carbonyl, amide and urethane groups; R⁶ =hydrogen or substitutedor unsubstituted C₁ to C₁₈ alkyl or C₆ to C₁₀ aryl; X=a hydrolysablegroup, namely halogen, hydroxy, alkoxy or acyloxy; Y=O or S; a=1, 2 or3; b=1, 2 or 3; c=1 or 2; and x=1, 2 or 3; and with the proviso that(i)a+x=2, 3 or 4 and (ii) a and/or b=1.
 2. An oxetane silane according toclaim 1, wherein at least one of the variables of formula (I), unlessotherwise stated, independently of the other variables, has thefollowing meaning:R⁰ =hydrogen or C₁ to C₅ alkyl; R¹ =C₁ to C₈ alkylene,these radicals being able to be interrupted by at least one groupselected from ether, thioether, ester and urethane groups; R² =missingor represents C₁ to C₈ alkylene, these radicals being able to beinterrupted by at least one group selected from ether, thioether, ester,thioester, carbonyl, amide and urethane groups or being able to bearthese in the terminal position; R³ =missing or represents methyl, ethylor phenyl; R⁴ =missing or represents --CHR⁶ --CHR⁶ --, --S--R⁵ --,--Y--CO--NH--R⁵ -- or --CO--O--R⁵ --; R⁵ =C₁ to C₈ alkylene, theseradicals being able to be interrupted by at least one group selectedfrom ether, thioether, ester, carbonyl, amide and urethane groups; R⁶=hydrogen or C₁ to C₅ alkyl; X=methoxy, ethoxy or chloro; Y=O or S; a=1;b=1; c=1 or 2; x=2 or 3; and/or a+x=3.
 3. An oxetane silane according toclaim 1 wherein at least one of the variables a, b or c is
 1. 4. Acomposition comprising the oxetane silane according to claim 1.